As a method of joining mutually thin stainless steel pipes and the like, known is a method, as disclosed in Japanese Patent Publication No. 33449/1984 now Japanese Patent No. 1453790, in which a pipe having been processed to have a reduced diameter at an end portion is inserted into a pipe fitting, and then a circumferential surface of the pipe fitting is allowed to undergo plastic deformation using a compressing tool. However, this method suffers problems that it requires an increased number of processing steps since the end portion of the pipe to be joined must be processed to have a reduced diameter at the job site and that a sectional area of the flow path is slightly reduced at the reduced diameter portion to increase flow resistance consequently.
In Japanese Provisional Patent Publication No. 88939/1980 or Japanese Patent Publication No. 39873/1978, there is disclosed a method in which a pipe is directly inserted to an end of a pipe fitting, and a joint is subjected to deformation using a compressing tool to achieve joining of pipes. However, this method suffers a problem that a circumferential surface of the end portion of the pipe which is brought into contact with a liquid flowing therethrough is in close contact with an internal surface of the pipe fitting to cause a phenomenon of crevice corrosion at this portion where an occluded corrosion cell is formed and the corrosion proceeds, so that the joint may be corroded in a relatively short period if said pipe is of stainless steel.